xvii CTENOSTOMATA AND CHEILOSTOMATA 481 



Ctenostomes (Fig. 238) are characterised by an extreme delicacy 

 of habit. The zooecia in these cases are sharply marked off from 

 the stem. They are either cylindrical or ovoid, being commonly 

 attached by a very narrow base, so that in some species they 

 readily fall off, and may thus be completely absent in certain parts 

 of the colony. In such forms as Vesicularia spinosa, it requires 

 considerable experience to recognise a stern which has lost its 

 zooecia as being part of a Polyzoon. In Mimosella the zooecia 

 possess a remarkable power of movement on the stem, similar 

 to that possessed by the leaflets of the Sensitive Plant. 1 In 

 certain forms (Bowerbankia, Amatkia) the zooecia occur in groups 

 separated by intervals which are devoid of zooecia, but in other 

 cases they may have a more irregular arrangement. The collar 

 to which this group owes its name is by no means a conspicuous 

 feature. Its position when retracted has been shown in Fig. 

 234, while Fig. 238 further illustrates its relations. 



The Cheilostomata grow in a great variety of forms, and also 

 show a wide range of character in their zooecia. The orifice is 

 commonly surrounded by stiff spines (Fig. 257, p. 524), which 

 perhaps have the function of protecting the delicate polypides 

 from the sudden impact of foreign bodies. These spines may 

 attain an enormous development, as in Bicellaria ciliata, and some 

 forms of Electro, (Membranipora) pilosa (Fig. 256, A). 



The operculum is usually, though by no means always, a con- 

 spicuous feature of the Cheilostome zooecium. It is invariably 

 of chitinous consistency, and is more or less semicircular in out- 

 line, the straight edge forming a hinge on which the operculum 

 opens. In some cases the orifice is surrounded by a raised 

 margin or "peristome" (Fig. 255, B, C) ; the operculum is then 

 situated at the bottom of a depression of the surface, and may be 

 concealed from view. In others, in which the front wall of 

 the zooecium is membranous (Bugula, Fig. 233), the operculum 

 is merely a part of this membrane, and so is quite inconspicuous ; 

 and in cases of this kind the membranous wall may be protected 

 by an arched spine, the " fornix," developed from one side of the 

 zooecium (Fig. 254,/). The ovicells are commonly a conspicuous 

 feature of this group, although they are believed to differ funda- 

 mentally from those of Cyclostomata. They have the form of a 

 helmet-like covering overhanging the orifice (Figs. 240, 241), 



1 Hincks, Brit. Mar. Polyzoa, i. p. 558. 

 VOL. II 2 I 



